Thermally regulated heating elements of a wide variety of types have existed for some time. Most often these elements have utilized some form of feedback control system in which the temperature produced is sensed and the source of electrical energization to the heating element is controlled either in a continuous, proportional or step-wise switching fashion to achieve more-or-less constant temperature.
Our patent application Ser. No. 071,682 (now U.S. Pat. No. 4,256,945) describes and claims an electrically resistive heating element which is intrinsically self-regulating at a substantially constant temperature despite large changes in thermal load. Such heating element includes an electrically conductive, non-magnetic substrate member of high thermal and high electrically conductive material having over at least a portion of one of its surfaces, a generally thin layer of a magnetic material. The magnetic material is selected to have a maximum relative permeability greater than one below its Curie temperature, but a minimum relative permeability of substantially one above its Curie temperature. The result is that when the heating element is electrically coupled to a source of high frequency electric power, the heating element is heated by Joule heating with the current from such source being principally confined by the maximum permeability to the generally thin magnetic layer in accordance with the skin effect at temperatures below the Curie temperature of the magnetic layer, but spreading into the non-magnetic member as temperature rises to approach such Curie temperature and the relative permeability declines. This provides intrinsic autotemperature regulation about tne Curie temperature irrespective of localized variations and significant fluctuations in thermal load, without resort to complex feedback systems to control electrical energization.
As seen from the above, the element of our parent patent application requires the application thereto of an alternating current. There are, however, many uses of such a heating element which require that the heat dissipation area of the same, i.e., that surface area from which thermal energy is to be drawn, be electrically isolated from the electrical energy applied to the heating element to cause heating.